Antenna



ANTENNA Andrew Alford, Boston, Mass.

Application January 13, 1953, Serial No. 331,032

7 Claims. (Cl. 343-767) The present invention is a continuation in part of my copending applications Serial No. 641,692 filed January V 12, 1946, now abandoned, and Serial No. 640,690, filed January 12, 1946, now Patent No. 2,625,654.

The present case relates more particularly to an antenna which is centrally fed through a coaxial transmission line entering through one end of the antenna and feeding across the air gap of the antenna which has the shape of a cylinder shorted at either end.

The purpose of 'isuch an arrangement is to obtain an intense omnidirectional horizontal radiation when the cylinder is placed in a vertical position.

According to the present invention the length of the antenna from end to end is made sutliciently long to enable the antenna to radiate with the maximum intensity in an omnidirectional horizontal plane. The theory relating to the dimensioning of the vertical length of antenna is generally summarized in my copending application Serial No. 641,692, but more accurately stated in myapplication Serial No. 640,690, filed simultaneously therewith.

As a continuation of my application Serial No. 641,692, the same specification as set forth therein is included in the specification below. In addition thereto for more complete explanatory purposes parts of the specification Serial No. 640,690 have also been included.

This invention makes use of a balanced transmission line shunted by a distributed inductance such as a metal sheet, bent into a cylinder of any suitable shape, and which accelerates the phase velocity of propagation of waves along said transmission line. By short circuiting the transmission line at both ends and by feeding power at a point intermediate said ends, a substantially cophasal distribution of potential is obtained along the transmission line. The potential existing between the conductors of the transmission line causes the flow of circumferential currents on the outer surface of the cylinder.

The distribution of the radiation field is concentrated in the region of a plane at right angles to the axis of the cylinder, and passing through the center thereof when the antenna is in free space. Allowance must be made for reflection effects when they are substantial as may be the case when the antenna is mounted in the neighborhood of ground.

The antenna of the present invention, when so placed that its axis is vertical, is particularly adapted for broadcasting on ultrahigh frequencies and concentrates radiant power in directions near the horizontal while radiation nearly an equal field in all directions of thecompass.

, The broadcast antenna is of simple construction and has United States Patent Figure 1 diagrammatically illustrates one embodiment of the invention, and,

Figures 2-7 are explanatory of the functioning thereof.

1 and 2 are the edges of a bent sheet 3 formed into a cylinder having a circular or any other suitable cross section. Metal discs 4, 5 are in electrical contact with the cylinder and provide closures for its two ends. A conice centric transmission line 6 is connected to a source of ultrahigh frequency power 7 and is introduced through an opening in bottom disc 4 into the cylinder 3.. The outer conductor of line 6 is usually routed parallel to edges 1 and 2 and is in contact with metal sheet 3. At a point such as 8, which is in the central region of cylinder 3 and preferably half-way between plates 5 and 4, the inner conductor 9 of line 6 is connected to one edge, e. g. 2 of the cylinder. The outer conductor of line 6 is connected to the opposite edge 1.

When an ultrahigh frequency potential is applied between edges 1, 2, two traveling waves proceed along these edges toward discs 4 and 5 where they are reflected. Thus a standing wave distribution of potential is produced along edges 1, 2. When the diameter of the cylinder 3 is made less than .17 wavelength, and preferably between .14 and .12 wavelength, i. e. when the effective surface area of a cross section thereof is less than .0235) and preferably between 0154).: and .0115): where x is the wavelength in free space, the phase velocity of propagation along the edges 1, 2 is approximately two times greater than the velocity of light so that the distance of the first voltage minimum of standing wave on edges 1, 2 from the nearest short circuited end of the cylinder 3 is comparable with one wavelength of the'same frequency in space.

The width of slot 1, 2, while not critical, is preferably small, for example of the order of .023).

' The distance between 4 and 5 is preferably more than it but less than 3).. A 42-inch cylinder having a 3.7 inches inner diameter having a slot inch wide was successthe flow of circumferential currents on the outside surface of sheet 3 as is diagrammatically indicated by arrows 12, 13 and 14. The amplitudes of these currents are in proportion to the amplitude of potential ditferences between edges 1 and 2.

The circumferential currents are approximately in the same phase when the half wavelength of the standing wave as measured along the edges 1, 2 as distinguished from one-half of a spaced-wave length is greater than the distance from 8 to either end 4 or 5. Such voltage distribution will be referred to as condition A.

It is useful to distinguish between two other types of voltage distributions which will be referred to as conditionB and condition C.

Under condition B point 8 is not centrally located and The discontinuity in the occurs opposite point 8, because at this point the two sets of standing waves produced by 4 and 5 come together. The potentials must be equal but not the slopes.

It should also be noted that 15b is not the maximum and that the lower portion of 15 reaches a higher value at its maximum than the upper portion.

In this case the currents in cylinder 3 flow in opposite directions at different points along the cylinder as is illustrated by arrows 16, 17, clockwise, above points 1511 and 18, 19, counterclockwise below points 15a, as viewed in Fig. 2.

Under condition C the distance between 8 and either end 4 and 5 is greater than the phase or virtual half wavelength of the standing wave but less than one wavelength thereof, as is illustrated by'curve 20 in Figure 3.

Points 20a along curve 20 correspond to point 151': of curve 15 in Figure 2. Near these two points occur reversals in phase in the potential existing between edges 1 and 2.

The sudden change in slope at 20b corresponds to 15b and is located opposite point 8.

When the distance between the minima 20a is substantially less than a half virtual wavelength, the potential at 20b is lowerthan the maxima of the upper and lower portions of curve 20. As the distance between minima 20a is increased to one-half virtual wavelength, the voltage at the maxima of the two portions of curve 20 becomes substantially less than voltage at 20b.

In case C the directions of the currents are, as is indicated by arrows 21, clockwise, 22 and 23, counterclockwise, and 24, clockwise, as viewed in Figure 3. Again the points of current reversal correspond to the phase reversal points 20a.

When the antenna of Figure l is operated so that the standing wave distribution is in accordance with condition A, the radiation. pattern of-the antenna in a plane through the axis of the cylinder is as shown by curve 25, in Figure 4. The radiation pattern in the plane through the center of the cylinder and at right angles to its axis is as shown by curve 26 in Figure 5A. The power gain of the antenna under these conditions is approximately 2.5 in comparison with a half wavelength.' When the antenna is mounted so that the axis of the cylinder is vertical, then the waves radiated by the antenna are horizontally polarized.

The high gain of this antenna together with the nearly circular radiation pattern and other desirable characteristics make is particularly well suited for ultrahigh frequency broadcasting. One antenna of Figure 1 has greater gain than an array of two stacked loop antennas. Two antennas of Figure 1 when arranged in a stacked array result in greater power gain than an array of four stacked loop antennas. This saving in the number of antennas required to produce a desired value of power gain results in a substantial saving in the number of branch lines, matching devices, insulators and in time and labor required to tune the stacked array.

When antenna of Figure l is operated in accordance with condition B the radiation pattern in space has the shape of a cone with maximum of radiation at an acute angle from the axis of the cylinder as is illustrated by curve 27 in Figure 5.

When the antenna of Figure l is operated in accordance with condition C there is a maximum of radiation in the plane at right'angles to the axis of the antenna in addition to other maxima which may be major or minor depending on the length of the cylinder with respect to the phase half wavelength as measured along the edges 1, 2. When the distance from point 8 to ends 4 and 5 exceed the phase half wavelength by a small amount, the main lobe of radiation is in the plane at right angles to the axis of the cylinder. When the distances between 8 and ends 4 and 5 exceed thephase half slope of curve 15 at 15b wavelength by a large amount, the minor lobes increase at the expense of th major lobe.

The elfect of the cylinder comprising the metal sheet 100, when properly proportioned, is to increase the phase velocity of the wave propagating along transmission line 110, 120, so that the distribution of voltage is substantially as indicated by dotted line 150. The distance of the point such as 160 which is the voltage maximum from the short circuit end 13 is accordingly greater than the space one-quarter wavelength of the output of ultrahigh frequency generator 140. The exact distance between 160 and the short circuit end 130 depends upon the inductance of the metal sheet per unit length of transmission line 110, 120. As the diameter of the metal sheet is decreased, the shunt inductance per unit length is also decreased and the distance between the point 130 and 160 is increased until a certain critical diameter of the cyhnder is reached. Preferably this distance should be greater than /5 of the space wavelength as long as the voltage distribution curve 150 of Figure 6 remains convex and does not become concave, under which conditions the damping of the wave traveling along the line would detrimentally effect the radiation.

When the distributed inductance per unit length has a reactance lower than the reactance of the distributed capacity between conductors and 120, then the transmission line degenerates into one which has a general equivalent to a transmission line which is strongly attenuated.

The principle indicated as applicable to one half of the antenna, applies with equal force to each half of the antenna centrally fed and follows the relationship shown in Figure l. The longer the cylinder may be made without the curve 11 becoming concave, the greater the radiation from the antenna.

As stated in the prior copending application, this distance depends upon the velocity of propagation along the cylinder as compared to the free space propagation. A value of twice the free space propagation of light or electro-magnetic waves will provide marked increased etfect in concentration and intensity of the horizontally polarized waves. The distance between the point of feed across the gap and the points of short circuit on the gap should not be less than Vs of a free space wave length.

It is however desirable that the relative amplitude along the whole cylinder providing the circulatory current from one side of the gap to the other, should be throughout the cylinder in the same direction and of amplitudes which do not freely vary from one end of the cylinder to the other.

A study of the relations of amplitudes along the transmission line is shown in Figure 7. At a frequency somewhat below the design frequency for which the antenna is to operate, the voltage distribution is substantially exponential as shown by 45 in Figure 7 labelled low frequency." As the frequency is increased, the voltage distribution changes progressively along the line by curves 55, 56 and 57. The curve 45 is obtained at the lowest frequency and curve 57 is obtained at the highest frequency.

The distribution of potential shown by curves 55 and 56 are more desirable than those shown by curves 45 and 57 because they approach more closely to the ideal distribution on which the distance of potential and, therefore, also the amplitude of the circumferential currents are more nearly uniform throughout the length of the cylinder. It will be noted that the potential at the minima 58 and 59 respectively are not less than /a of the maxima 60 and 61 of the same respective curves. The minimum 62 of curve 57 is very low. By operating near the cut-off frequency, that is, with the potential distribution such as 55 or 56, it is possible to achieve a greater gain because the currents are more nearly uniform along the length of the antenna and because the antenna may be 53, where the phase begins to rise rapidly. The phase of the currents near the potential minimum is approximately in quadrature with the large currents at other points closer to the short-circuited end. If such currents are allowed to flow they radiate power which is advanced in phase with respect to the power radiated by currents along other portions of the antenna producing an undesirable tilt of the maximum radiation from the plane perpendicular to the cylinder and passing through it. This undesirable effect is avoided by making the distance from the feed point to the short-circuited end of the cylinder just a little short of the distance from the shortcircuited end to the point of minimum as illustrated by curves 55 of Figure 7 and 150 of Figure 6.

As set' forth in my companion application S. N. 640,690, this distance is approximately K of the distance between the short-circuited end and the minima, referred to the virtual half wavelength.

Referring to Figure 7 which assumes a cylinder having a circular cross section and a gap equal to Vs of its diameter, it will be seen that as the frequency is decreased the attenuation increases. The standing waves are superposed on an exponential attenuation curve so that as the frequency is reduced the voltage minimum rises higher and higher until there is no longer a minimum. Finally the attenuation becomes so rapid that the wave from the feeder does not reach the short-circuited end, and there are no standing waves.

The distribution of phase of the voltage across the gap and the voltage amplitude are also shown in Figure 7. The phase curves 50, 51, also show that the phases of the circumferential currents at particular sections differ only by a constant amount from the phase of the potential across the gap at this section. q

The phase curve 50 has a shelf portion 52, 53 along which the phase remains approximately constant, around 90. At the point of minimum of potential the phase is advanced by about 75 with respect to the phase along the shelf. Still farther from the short-circuited end the phase advances rapidly. When the attenuation is reduced as in the case of curve 57 similar .phenomena take place except that the phase error at the voltage minimum approaches closer to 90'.

It will be clear from these curves that the cylinder from. the feed point to the short-circuited end should not be longer than about .9 of the distance from the shortcircuited end to the minimum because, otherwise, currents near the open end of the cylinder, being advanced in phase, will tend to tilt the minimum of radiation away from the plane perpendicular to the axis of the antenna.

The length of the cylinder may. be increased near the cut-ofi frequency. Since at a given frequency the power gain increases even somewhat faster than the length of age distribution such as is shown by curve 55 or 56. The voltage distribution 56, while still usable, is not as efiicient as 55. The voltage distribution shown by curve 57 is not very efficient.

The eflicient voltage distributionis one in which voltage minimum is not less than .3 of the maximum and preferably not less than .5 of the maxima.

Having now described my invention, 1 claim:

1. An antenna for radiating horizontally polarized high frequency radio waves of a given band comprising a conducting cylinder having a longitudinally extending slot, means for feeding the cylinder with a transmission line with one terminal connected adjacent the slot at a point on one side and the other terminal adjacent the slot at a point sustantially opposite on the the other side, short circuit means across said slot spaced from both sides of said connecting point, the distance from said short circuit means to said connecting points being not substantially greater than .9 of the standing half wavelength along the cylinder with' the internal cross sectional area of the cylinder being between .0235). and .0115). where A is the wave length corresponding to'a frequency within the band, said cylinder having a potential distribution curve from the point of feed connections in both directions along said tube which is convex and said conducting cylinder having a length substantially not less than 1.5 free space wave lengths long between the short-circuit means.

the cylinder, it is very desirable to operate with a volt- 2. An arrangement as set forth in claim 1 in which the transmission line passes through one end of the cylinder.

3. An arrangement as set forth in claim 2 in which said cylinder is fed by a coaxial transmission line having the outer conductor connected adjacent one side of the slot in the cylinder and the inner conductor connected. to the other side of the slot adjacent the first connection.

4. An arrangement as set forth in claim 2 in which said cylinder has a cross sectional area between .015). and :011571.

5. An arrangement as set forth in claim 1 in which said short circuit means are adjustable along said slot conforming to the dimensions as set forth therein.

6. An arrangement as, set forth in claim 1 in which the potential phase velocity along the slot is substantially the cylinder is circular with an inner diameter of 0.138

space wave lengths and the length between short circuit means is 1.57 space wave lengths'where the space wave length corresponds to a frequency in the operating band.

References Cited in the file of this patent UNITED STATES PATENTS 2,414,266 Lindenbald Jan. 14, 1947 2,489,288 Hansen Nov. 29, 1949 2,513,007 Darling June 27, 1950 

